63 + Interview Questions in HEAT TRANSFER IN GENERAL AWARENESS Page 2 InterviewSolution

51.	Give reason for the increase in rate of evaporation of a liquid when (a) air is blown above the liquid (b) surface area of liquid is increased (c) temperature of liquid is increased.
Answer» (a) BLOWING AIR ON LIQUID SURFACE INCREASES RATE OF EVAPORATION: Blowing air takes away with it the molecules of liquid escaping out of the surface. To take their place, other molecules escape out from the surface of liquid. (b) SURFACE AREA INCREASES THE RATE OF EVAPORATION: On increasing the area of the surface, a number of molecules escaping out from the surface increases. (c) INCREASE IN TEMPERATURE INCREASES THE RATE OF EVAPORATION: Increase in temperature increases K.E. (K.E = 1/2 Mv²). More and more molecules come to the surface of liquid hence the rate of evaporation will increase with an increase in temperature.

51.

Give reason for the increase in rate of evaporation of a liquid when (a) air is blown above the liquid (b) surface area of liquid is increased (c) temperature of liquid is increased.

Answer»

(a) BLOWING AIR ON LIQUID SURFACE INCREASES RATE OF EVAPORATION: Blowing air takes away with it the molecules of liquid escaping out of the surface. To take their place, other molecules escape out from the surface of liquid.

(b) SURFACE AREA INCREASES THE RATE OF EVAPORATION: On increasing the area of the surface, a number of molecules escaping out from the surface increases.

(c) INCREASE IN TEMPERATURE INCREASES THE RATE OF EVAPORATION: Increase in temperature increases K.E. (K.E = 1/2 Mv²). More and more molecules come to the surface of liquid hence the rate of evaporation will increase with an increase in temperature.

Discussion

52.	Select the correct alternativeIn evaporation 1. all molecules of liquid begin to escape out 2. only the molecules at the surface escape out 3. the temperature of liquid rises by absorbing heat from surroundings. 4. the molecules get attracted within the liquid.
Answer» In evaporation, only the molecules at the surface escape out.

Discussion

53.	Write true or false for the statementAll molecules of a liquid take part in the process of evaporation.
Answer» False All molecules of a liquid take part in the process of evaporation.

Discussion

54.

Name two ways of change of liquid state to the vapor state and distinguish them.

Answer»

Two ways of change of liquid state to vapor state are:

(i) Evaporation

(ii) Boiling.

DIFFERENCE BETWEEN EVAPORATION AND BOILING:

BOILING	EVAPORATION
(i) Heat is supplied	(i) Heat is absorbed.
(ii) Fast process.	(ii) Slow process.
(iii) Starts from bottom.	(iii) Start from surface i.e surface phenomenon
(iv) Sound is produced.	(iv) Silent process
(v) Heating effect	(v) Cooling effect.
(vi) Takes places at a fixed temperature called boiling point.	(vi) Takes places at all temperatures.

Discussion

55.	No heat is supplied to a liquid during evaporation. How does then the liquid change into its vapors ?
Answer» Though no heat is supplied to the liquid but molecules near the surface of the liquid acquire sufficient kinetic energy by collisions with other liquid molecules and with this K.E. they overcome the attractive forces of other molecules and change into vapors.

Discussion

56.	How is the heat energy supplied to a liquid used during boiling at a fixed temperature ?
Answer» When heat is supplied, temperature of liquid rises continuously until the water starts boiling at 100°C. Once the water starts boiling, its temperature does not rise further, although heat is still being supplied. This heat is being used to change each and every water molecule into vapor.

Discussion

57.	What is the change in average kinetic energy of molecules of a liquid during boiling at its boiling point ?
Answer» AVERAGE KINETIC ENERGY is the measure of the temperature of body. When the temperature of body increases the average kinetic energy increases and when Average Kinetic energy decreases there is decrease of body temperature and hence at boiling point average Kinetic energy increases.

Discussion

58.	The boiling point of a liquid is increased by 1. increasing the volume of liquid 2. increasing the pressure, on liquid 3. adding ice to the liquid 4. decreasing pressure on liquid.
Answer» The boiling point of a liquid is increased by increasing the pressure, on liquid.

Discussion

59.	Two bodies A and B having equal surface areas are maintained at temperatures 10°C and 20°C. The thermal radiation emitted in a given time by A and B are in the ratio (a) 1 : 1.15 (b) 1 : 2 (c) 1:4 (d) 1 : 16.
Answer» The correct answer is (a) 1 : 1.15

Discussion

60.	Two identical rods of copper are heated to different temperatures — one by 5°C and the other by 10°C. Which rod will expand more ?
Answer» Two rods with same length and material are heated to different temperature. L_t - L₀ ∝ t ∴ Rod with higher temperature i. e. at 10°C will expand more.

60.

Two identical rods of copper are heated to different temperatures — one by 5°C and the other by 10°C. Which rod will expand more ?

Answer»

Two rods with same length and material are heated to different temperature.

L_t - L₀ ∝ t

∴ Rod with higher temperature i. e. at 10°C will expand more.

Discussion

61.	A steel frame (K = 45W/m-°C) of total length 60cm and - cross-sectional area 0.20cm2, forms three sides of a square. The free ends are maintained at 20°C and 40°C. Find the rate of heat flow through a cross-section of the frame.
Answer» K = 45w/m-°C ℓ = 60cm = 60 × 10^–2m A = 0.2cm² = 0.2 × 10^–4m² Rate of heat flow, = KA( θ₁ + θ₂ )/l = (45 x 0.2 x 10^-4 x 20)/(60 x 10^-2) = 30 x 10^-3 = 0.03w

Discussion

62.	In a room containing air, heat can go from one place to another(a) by conduction only (b) by convection only(c) by radiation only (d) by all the three modes.
Answer» (d) by all the three modes.

Discussion

63.	An icebox almost completely filled with ice at 0°C is dipped into an area volume of water at 20°C. The box has walls of surface area 2400cm2, thickness 2.0mm and thermal conductivity 0.06W/m-°C. Calculate the rate at which the ice melts in the box. Latent heat of fusion of ice = 3.4 x 105J/kg.
Answer» A = 2400cm² = 2400 × 10^–4m² ℓ = 2mm = 2 × 10^–3m K = 0.06w/m-°C θ₁ = 20°C θ₂ = 0°C Q/t = (KA(θ₁ - θ₂)/l = (0.06 x 2400 x 10^-4 x 20)/(2 x 10^-3) = 24 x 6 x 10^-1 x 10 = 24 x 6 = 144J/sec Rate in which ice melts = m/t = Q/(t x L) = 144/(3.4 x 10⁵)kg/h = (144 x 3600)/(3.4 x 10⁵)kg/s = 1.52kg/s.

63.

An icebox almost completely filled with ice at 0°C is dipped into an area volume of water at 20°C. The box has walls of surface area 2400cm2, thickness 2.0mm and thermal conductivity 0.06W/m-°C. Calculate the rate at which the ice melts in the box. Latent heat of fusion of ice = 3.4 x 105J/kg.

Answer»

A = 2400cm² = 2400 × 10^–4m²
ℓ = 2mm = 2 × 10^–3m
K = 0.06w/m-°C
θ₁ = 20°C
θ₂ = 0°C

Q/t = (KA(θ₁ - θ₂)/l = (0.06 x 2400 x 10^-4 x 20)/(2 x 10^-3) = 24 x 6 x 10^-1 x 10 = 24 x 6 = 144J/sec

Rate in which ice melts = m/t = Q/(t x L) = 144/(3.4 x 10⁵)kg/h = (144 x 3600)/(3.4 x 10⁵)kg/s = 1.52kg/s.

Discussion

Explore topic-wise InterviewSolutions in .

Give reason for the increase in rate of evaporation of a liquid when (a) air is blown above the liquid (b) surface area of liquid is increased (c) temperature of liquid is increased.

Select the correct alternativeIn evaporation 1. all molecules of liquid begin to escape out 2. only the molecules at the surface escape out 3. the temperature of liquid rises by absorbing heat from surroundings. 4. the molecules get attracted within the liquid.

Write true or false for the statementAll molecules of a liquid take part in the process of evaporation.

Name two ways of change of liquid state to the vapor state and distinguish them.

No heat is supplied to a liquid during evaporation. How does then the liquid change into its vapors ?

How is the heat energy supplied to a liquid used during boiling at a fixed temperature ?

What is the change in average kinetic energy of molecules of a liquid during boiling at its boiling point ?

The boiling point of a liquid is increased by 1. increasing the volume of liquid 2. increasing the pressure, on liquid 3. adding ice to the liquid 4. decreasing pressure on liquid.

Two bodies A and B having equal surface areas are maintained at temperatures 10°C and 20°C. The thermal radiation emitted in a given time by A and B are in the ratio (a) 1 : 1.15 (b) 1 : 2 (c) 1:4 (d) 1 : 16.

Two identical rods of copper are heated to different temperatures — one by 5°C and the other by 10°C. Which rod will expand more ?

A steel frame (K = 45W/m-°C) of total length 60cm and - cross-sectional area 0.20cm2, forms three sides of a square. The free ends are maintained at 20°C and 40°C. Find the rate of heat flow through a cross-section of the frame.

In a room containing air, heat can go from one place to another(a) by conduction only (b) by convection only(c) by radiation only (d) by all the three modes.

An icebox almost completely filled with ice at 0°C is dipped into an area volume of water at 20°C. The box has walls of surface area 2400cm2, thickness 2.0mm and thermal conductivity 0.06W/m-°C. Calculate the rate at which the ice melts in the box. Latent heat of fusion of ice = 3.4 x 105J/kg.